With increasing attention toward carbon-neutral energy production, solar electricity—or photovoltaic (PV) technology—is receiving heightened attention as a potentially widespread approach to sustainable energy production. The current PV technology is based largely on the use of crystalline silicon wafers. It has proved very difficult with this technology to reduce the total system cost down to the level needed to achieve widespread adoption of this technology for energy production.
Organic photovoltaic (OPV) technology is an attractive alternative to silicon-based solar electric conversion. Advancements in organic solar cells and OLEDs include processing advantages that promise lower production costs and simpler fabrication methods when compared to their inorganic counterparts. Furthermore, organic solar cells (OSCs) offer the possibility of device fabrication on flexible substrates over large areas with higher throughput, which could greatly improve both their functionality and economy.
More specifically, polymer and small-molecule based OPVs have gained a lot of attention in recent years due to their potential low cost and roll-to-roll manufacturing capability, and amenability to flexible substrates. Some of the developments that have improved performance of OPVs are based on electron donor-acceptor heterojunctions. In a planar heterojunction, or ‘bilayer’ device, excitons are dissociated into charge-carriers at the donor-acceptor interface. The efficiency of PV conversion is, however, low because only the excitons created within the exciton-diffusion length from the donor-acceptor interface are utilized.
The introduction of bulk-heterojunction (BHJ) OPVs, in which the donor-acceptor materials are blended together, resulted in efficiency improvements for organic solar cells. If the length scale of the phase separation in the donor-acceptor blend is similar to the exciton-diffusion length, then all of the excitons photo-generated in either material can potentially diffuse to an interface and dissociate into free charge carriers. These charge carriers may then drift to their respective electrodes if continuous pathways exist in each material. The electron acceptors are often the fullerenes or their derivative, e.g., [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) which have relatively high miscibility in organic solvents compared to other known acceptor materials. For OPVs with poly(3-hexylthiophene) (P3HT) as the electron donor, efficiencies of about 5% have been realized.
However, the performance of these bulk heterojunction devices is still limited by several factors. The high energy band gap of most polymer materials poses a limitation on the capability to harvest lower energy photons from sunlight. Moreover, the charge carrier mobility of these materials is generally low, making it necessary to keep the thickness of the active layer low. A thinner film between the electrodes can lower the probability for charge recombination which lowers device efficiency, and can also increase the carrier drift velocity due to higher electric field. However, the optical absorption will be low in such thin films. Thus, there is a conflict between the optical length scale and the electronic length scale. Another problem is charge recombination. Electron-hole pairs generated within the donor polymer on absorption of light still recombine if they do not find an acceptor interface within their lifetime. These electron-hole pairs are called singlet excitons. Even if these singlet excitons dissociate, the electron on the acceptor and the hole on the donor form charge-transfer excitons, which can again recombine. Thus, recombination of singlet and charge-transfer excitons is one of the biggest loss mechanism in modern organic solar cells.
It would therefore be desirable to have a method of fabricating organic photovoltaic devices that allows for increased dissociation of or reduced recombination of singlet excitons and charge-transfer excitons. Embodiments of the invention provides such a device. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.